1. Field of the Invention
This invention relates to rotary cone rock bits having hard metal cutter inserts strategically positioned within the rotary cones of the rock bit.
More particularly, this invention relates to inclined chisel inserts used particularly in a gage row of a rotary cone for a rock bit.
2. Description of the Prior Art
There are a number of prior art patents that disclose inserts that have certain non-symmetric features. For example, U.S. Pat. No. 3,442,342 discloses a rotary cone rock bit having tungsten carbide chisel inserts in a gage row of each of the three cones. After the bit is assembled, the sides of the gage row inserts are ground flat to the precise gage diameter of the hole to be drilled. The gage row inserts are intentionally installed so that the rock bit, when all three cones are in position, is overgage. The gage row inserts then have to be ground to provide a flat surface so that the diameter of the bit is correct. The patent goes on to teach that if there were no flats on the gage row inserts and the convex surface were simply tangent to a side wall of a borehole, there would be nothing but point contact and the borehole would quickly become undergage as the contact points of the inserts wore away.
It has been determined, however, that gage chisel type inserts having flat spots ground therein provide a relatively large contact area against the borehole sides. Each of the inserts then can be susceptible to heat checking, resulting in premature wear and/or insert breakage. Insert heat checking can be defined as high cycle thermal fatigue due to intermittent frictional heat generated by borehole wall to gage insert contact and subsequent cooling by drilling fluid per each revolution. Certain formations such as shales can generate inordinate amounts of frictional heat at the borehole wall/gage insert interface. If the cobalt contents of the tungsten carbide alloy inserts is reduced or the tungsten carbide grain size is adjusted to reduce the tendency to heat check (independent of geometry change), then typically, the fracture toughness of the insert is reduced and the design is more susceptible to pure mechanical fatigue failure.
U.S. Pat. No. 4,058,177 describes a non-symmetric gage row insert which provide a large wall contacting surface supposedly decreasing the wear on the gage insert because of the larger contact area and increasing the ability of the earth boring apparatus to maintain a full gage hole. The insert has a shape prior to assembly onto the rock bit apparatus that includes a base integrally joined to a non-symmetric head. The base is mounted within the cone and the head projects from the rock bit cone and includes an extended gage cutting surface that is flat. The gage cutting surface contacts the wall of the hole with the majority of the length of its extended surface.
This patent, like the foregoing patent, provides a gage row insert with a large flat surface that parallels the borehole wall and thus is subject to the same insert degradation as the foregoing patent.
Another U.S. Pat. No. 4,108,260, describes specially shaped non-symmetrical inserts to be used in rotary cone rock bits. The insert is generally chisel-shaped with flanks converging to a crest. The flanks are non-symmetrical with respect to each other, the leading flank is scoop-shaped and the trailing flank is rounded outwardly. This insert is designed for increased penetration in a rock formation. The insert is not, however, designed specifically for a gage row of a rock bit to maintain the gage of the bit as it is used in a borehole.
Still another prior art U.S. Pat. No. 4,334,586, describes inserts for drilling bits. The insert cutting elements comprise non-symmetrical inserts placed in at least one circumferential row in a roller cone in alternating alignment. This non-symmetrical type insert is cone-shaped with the apex of the insert rounded and off-center. Each insert in the circumferential row is alternated so that its apex is not aligned with its neighboring insert, every other insert being so arranged in rows on a rotary cone of a rock bit.
This non-symmetrical insert, like the foregoing insert, is not designed to be placed in a gage row of a cone to provide maximum gage protection during bit operation in a borehole.
The foregoing prior art patents are disadvantaged, especially those patents that teach a flattened area to be positioned adjacent a gage row of a rotary cone. The large area flat surface paralleling the wall of a borehole makes the gage row inserts susceptible to heat checking thereby prematurely wearing the insert and, in many cases, causing the insert to fracture through thermal fatigue failure. When this occurs the rock bit quickly goes undergage, creating all kinds of problems for subsequent new bits that are placed back into the borehole for further penetration of a formation. If a dull bit is undergage when removed or "tripped" from the borehole, a following new full gage bit will immediately pinch, forcing the cones inwardly towards each other and rendering the bit useless thereafter. The remedy is a costly reaming operation to bring the borehole back to gage.
Symmetrical chisel type inserts are sometimes used on gage and they do provide a conical rather than flat gage cutting surface adjacent to the borehole wall. However, the cutting surface of these inserts often does not closely parallel the borehole wall, therefore allowing the bit to go undergage much earlier. When the cone angle of a standard chisel insert is increased to improve the gage surface angle (or the angle between the side of the cone and the borehole wall), the extension of the insert becomes limited because the crest length decreases as the insert extension increases. Therefore, a special non-symmetrical insert is designed to provide increased crest length while providing the desired gage surface angle, thus providing maximum gage-keeping capability while minimizing wear on the special non-symmetric inserts as taught in the present invention. It has been found that conical-shaped gage cutting surfaces provide a more desirable line or point contact rather than a full surface, large area contact like a gage chisel insert having a flat side as indicated in the foregoing prior art. The conically shaped gage cutting surface reduces the possibility of heat checking that can lead to catastrophic failure of the insert. In other words, it is desirable to have a design balance between the thermal fatigue associated with heat checking and the mechanical fatigue associated with insert shape and respective strength.